This invention relates to detecting the leading edge of media as it advances through a printer.
Ink jet printers include various kinds of apparatus for detecting the presence and position of the leading edge of print media, such as a sheet of paper, as the media advances through the printer. Identifying the position of the leading edge of the media is an important step because it is one factor necessary to ensure high quality printing. Thus, when the leading edge of a sheet of paper is advanced past the leading edge detector, a zero point reference signal is typically generated for positioning an image correctly on the media. The zero point reference signal may be an on/off signal that indicates to the printer controller that the leading edge of print media is present, and identifies the position of the leading edge. Through the controller, this reference signal initiates a series of events such as a counting sequence that, among other things, correlates to the position on the media at which ink may begin to be deposited. Since the media is moving through the printer, the counting sequence is one part of the determination of where on the page printing begins.
Ink jet printers include a carriage that may hold one or more ink-filled print cartridges. The carriage reciprocates in a back and forth motion across the printing surface, positioning the ink cartridges adjacent the media for printing. During the printing operation the carriage is shuttled across the paper and ink droplets are ejected out of the cartridge onto the paper in a controlled manner to form a swath of an image each time the carriage is scanned across the page. Between carriage scans, the paper is advanced with a media feed assembly so that the next swath of the image may be printed. Sometimes, more than one swath is printed before the paper is advanced. In some printers, a stationary print head or array of print heads may be provided to extend across the entire width of the paper that moves through the printer.
The relative position of the print head(s) and paper must be precisely maintained to effect high-resolution, high-quality printing. Paper advancement past the print head, and carriage drive functions are typically separately controlled. As to the former, the paper advancement assembly typically includes friction rollers or tractor feed mechanisms that advance the recording media past a xe2x80x9cprint zone.xe2x80x9d With an ink jet printer, in the course of advancing the print media between swaths, a disk encoder and associated servo systems are one of the usual methods typically employed for controlling the precise incremental advance of the media. This incremental advance is commonly called xe2x80x9clinefeed.xe2x80x9d Precise control of the amount of the advance, the linefeed distance, is critical for high print quality.
Likewise, the position of the carriage as it reciprocates in a direction transverse to the direction that the paper is fed through the printer must be precisely controlled. Typically, the carriage assembly includes an optical sensor or encoder carried on the carriage such that it is positioned adjacent to - - - typically encircling - - - an encoder strip that extends laterally across the printer. A servo system is used in concert with the encoder and encoder strip to precisely control the position of the carriage relative to the media - - - typically by moving the carriage along a carriage shaft with a continuous drive belt.
The printer controller controls and synchronizes both the reciprocating movement of the carriage, and the linefeed so that ink is deposited in a desired manner on the media.
Detection of the leading edge of media as it advances through the printer is an important component of the printing process because the printer controller relies upon the signal generated by the leading edge to determine the position on the page where printing may begin. For this reason, it is important that the printer controller is informed of the presence and position of the leading edge of the print media so that as the media is advanced past the carriage, ink in the first swath is deposited at precisely the correct location on the page. Many printers utilize separate detectors to perform this so-called xe2x80x9cleading edgexe2x80x9d or xe2x80x9ctop of formxe2x80x9d sensing. These detectors often are relatively expensive units such as optical sensors or through-beam type sensors that are dedicated to the job of sensing the present of the media leading edge and transmitting a reference signal to the printer controller. In the case of optical sensors, when an optical beam is interrupted by the leading edge of the paper or the media activates a mechanical xe2x80x9cflagxe2x80x9d, the reference signal is generated and transmitted to the controller.
Electro-optical sensors like those described are typically relatively sophisticated and complicated parts that require the use of dedicated hardware such as wiring and cabling, and dedicated input/output on the ASIC controlling the printer. In addition to relative complexity, such sensors can be relatively expensive. Although conventional top of form sensors like those just described function adequately to inform the printer controller of the presence of the media leading edge, given their relative complexity and cost, they also present an opportunity for simplifying printer structure and reducing printer costs by replacing those sensors with simplified apparatus for detecting the leading edge of media advancing through the printer.
The present invention is generally directed to techniques for top of form sensing - - - that is, detecting the leading edge of media as it is advanced through a printer. Rather than relying upon hardware dedicated to the single function of detecting the media leading edge to generate the zero reference point signal, the invention relies upon hardware that is already present in the printer but used for other purposes. In doing so, the top of form sensor of the present invention eliminates costly hardware dedicated to the single function of top of form sensing and simplifies printer structure and operation.
In one approach to the invention, the carriage axis encoder strip that is already incorporated into the printer in connection with the print cartridge carriage is utilized to generate the zero point reference signal upon detection of the media leading edge.
In one embodiment, a mechanical sensor mechanism detects the media leading edge and causes a corresponding signal change in the carriage axis encoder. The controller interprets the signal change to correspond to the presence of the media leading edge. The invention thus relies upon the functionality of existing printer parts to accomplish a task that previously required additional hardware. By relying upon existing parts the costs associated with separate leading edge sensors may be eliminated, thereby simplifying printer construction and operation, and reducing the overall cost of the printer.
In one embodiment, the sensor mechanism comprises a lever that interrupts the media path when no media is present in the printer. When media is advanced through the printer along the media path, the leading edge of the media is advanced into contact with the lever. As the leading edge of the media is advanced into contact with the lever, the lever operates a hammer that contacts the encoder strip. Movement of the encoder strip caused by the touch of the hammer generates a reference signal that is transmitted to the controller corresponding to the presence of the media leading edge.
Apparatus and methods for carrying out the invention are described below. Other advantages and features of the present invention will become clear upon review of the following portions of this specification and the drawings.